1. Field of the Invention
The present invention relates to an optical displacement measuring apparatus, and, more particularly, to an optical displacement measuring apparatus for introducing light from a light source through an objective optical system to an object to be measured, and detecting light reflected from the object to be measured in a detection unit to measure a displacement of the object to be measured relative to the objective optical system.
2. Description of the Related Art
Japanese Patent Publications JP 2005-265616 and JP 2006-58115 describe, as means for measuring a displacement of an object in a non-contact manner, conventional technologies such as “a triangulation system using light”, “an optical probe method”, and “an automatic focusing technique”.
Here, in the “triangulation system using light”, a light beam is irradiated from a semiconductor laser or the like to a surface to be measured, and light reflected from the irradiated surface is imaged by means of a condenser lens onto an optical position detector placed in a direction which differs from an irradiation direction. According to this method, because the movement of the surface brings about changes in position of the beam on the position detector, a displacement of the surface to be measured can be determined rapidly according to a principle of a triangulation method. However, on the principle of the triangulation method, it is necessary for the light source to be placed at a sufficient distance from the optical position detector in order to realize high-precision measurement. Accordingly, because sufficient space available for measurement is needed in the vicinity of the surface to be measured, the triangulation system using light is unfit for use in displacement measurement of an object such as a workpiece housed in a casing having a small mouth.
The “optical probe method” includes a critical angle method, an astigmatic method, a knife edge method, a heterodyne method, and other methods. In any of these methods, minute spot light is imaged on an object, and reflected light from the object is collected to detect a condition of the reflected light. Thus, these methods can achieve highly sensitive detection, and apparatuses employing these methods can be designed as a small and light-weight configuration and may be used, for example, as an optical probe for optical measurement of surface roughness. However, because a detection range with high precision is limited to an area close to a focused position, this method suffers from a very narrow measurable range. In general, the measurement is available only within, for example, a range of several micrometers. In addition, the method also has a problem that a measured value varies depending on a surface condition of the object.
In the “automatic focusing technique”, because focusing is always obtained on an object by moving a lens, a displacement of the object can be measured from a travel distance of the lens. Therefore, this technique can realize a broader measurement range. However, in automatic focusing operation, for example, when a surface of the object has discontinuous displacements due to the presence of unevenness such as steps, the lens is at risk of losing its tracking direction, thereby necessitating a new search operation. Accordingly, it is, in some cases, difficult to perform high-speed measurement using the auto-focusing technique.
JP 2005-265616 discloses an optical displacement measurement device including a conical objective lens placed in front of an object and using optical properties as described below. Light emitted from behind the objective lens is introduced into the objective lens and refracted on a cone-shaped interface of the objective lens to irradiate the object, and light reflected from the object is returned to the objective lens, refracted again by the cone-shaped interface, and delivered in parallel to the light which is initially emitted. It is further described that because the light returned from the objective lens is offset from a central optical axis of the objective lens according to a distance between the object and the objective lens, i.e. an amount corresponding to a displacement of the object, the displacement of the object can be measured based on an amount of the offset.
On the other hand, JP 2006-58115 discloses an optical displacement measurement device in which a conical objective prism is placed in front of an object to be measured, and outbound light shifted in parallel with a central optical axis of the objective prism is introduced from behind the objective prism to the object. Also in this case, the outbound light having passed through the objective prism is reflected from the object and returned to the objective prism through which the light is converted into inbound light which is parallel to the initially emitted outbound light. An offset amount of the inbound light from the central optical axis varies depending on the displacement of the object to be measured. It is further described that the inbound light is focused by a condenser lens at a focus point, to thereby suppress effects of scattered light by means of a pinhole optical device disposed at the focus point.
When the conical objective optical systems described in the above-described Publications JP 2005-265616 and JP 2006-58115 are used, an end section can be made compact in size, so that a displacement of a workpiece housed in a casing whose mouth is small can be measured. In addition, mechanical operation is not needed, and a wide range of measurement can be realized at high speed. However, in the teachings of the above-described Publications, light introduced through the conical objective optical system to the object to be measured is obliquely irradiated on the object. The use of such oblique incident light may result in the occurrence of an event that when the object to be measured has an uneven surface including, for example, bumps and dips or elevation changes, a position struck by the incident light varies depending on the displacement of the object to be measured, which would introduce an error into a displacement measuring result.